Conventionally, a current transformer has been widely used as a current sensor, but its low sensitivity requires a laminated iron core, and the iron core generates magnetic saturation, thereby causing the problem of an insufficient current detection range. The iron core also causes the problem of a large sensor unit.
On the other hand, there is a method in which a Hall element and a magnetoresistive element are used as current detection elements. However, since they are low in detection sensitivity, the sensitivity is commonly improved by providing a magnet gathering core and a Hall element or a magnetic element mounted at the gain of the magnet gathering core.
Like the current transformer, the above-mentioned method of using the magnet gathering core uses a core of at least 3˜4 cm and requires a large sensor unit, and generates the magnetic saturation by the iron core, thereby obtaining an insufficient current detection range. Furthermore, since the Hall element and the magnetoresistive element have large output fluctuations depending on the temperature, a temperature compensating circuit is required.
A high-sensitive magnetic detection element for replacing the Hall element and the magnetoresistive element can be, for example, a magnetic impedance element of an amorphous wire disclosed by Japanese Patent Application Laid-open No. Hei 6-347489, and a thin film disclosed by Japanese Patent Application Laid-open No. Hei 8-73835.
A magnetic impedance element of any shape indicates a high-sensitive magnetic detection characteristic, but the magnetic impedance element of an element itself has nonlinearity like the example of the magnetic impedance characteristic by the amorphous wire shown in FIG. 10. By adding the bias magnetic field, the linearity of the dependence on the magnetic field to which an impedance variation is applied is improved (Japanese Patent Application Laid-open No. Hei 6-176930), a negative feedback coil is wound around the magnetic impedance element, a current proportional to the voltages on both ends of the magnetic impedance element is applied to the coil, and a negative feedback is provided, thereby obtaining an element excellent in linearity (Japanese Patent Application Laid-open No. Hei 6-347489).
The above-mentioned bias magnetic field is normally obtained by applying power to the coil wound around. However, in this case, two types of coils, that is, a bias coil and a feedback coil, are required, thereby upsizing the entire system.
Furthermore, using a wire type or a thin film type magnetic impedance element, there is the problem of variable element sensitivity depending on the material (magnetic permeability, resistivity, etc.) used when a magnetic impedance element is produced and the variance in element size (length, film thickness, film width, etc.).
FIG. 11 shows a common example of a detection circuit of the magnetic impedance element.
The detection circuit obtains impedance of a magnetic impedance element 1 by outputting through the detection circuit A and the amplification circuit B the output obtained when a high frequency current passes from a high frequency current generator (OSC) 4 to the magnetic impedance element 1. At this time, the output is adjusted by a variable resistor VR.
However, to reduce the variance in element sensitivity in the circuit, it is necessary to adjust and correct each system, thereby requiring a larger cost. Although each system can be adjusted and corrected, an automatic correction cannot be made. Therefore, the output of a device varies with time depending on the variations in temperature, etc., thereby causing the problem that high precision compensation cannot be realized.
Accordingly, the object of the present invention is to measure a wide current range with high precision using a small and low-cost system without reducing the precision by an environmental feature or with time.